The present invention relates to a coupling assembly and in particular to a coupling assembly suitable for releasably interconnecting fluid passages.
It is a common requirement in many industries to be able to quickly and releasably interconnect two passages (e.g. pipes or hoses) containing fluids. The range of fluids to be transported, together with their properties, may vary widely, including gases such as air within medical oxygen masks and liquids such as oil within undersea drilling operations. The pressure of the fluid passing through the coupling assembly may vary from substantially the same as the ambient pressure around the assembly in the case of oxygen masks to high-pressure liquids at pressures many times the ambient pressure in the case of oil pipelines.
A number of quick release coupling arrangements are known in the art whereby the ends of fluid passages are provided with corresponding coupling members to facilitate the joining of passages. This may be provided in the form of a female coupling member comprising a socket and a corresponding male coupling member comprising a probe receivable in the socket. The coupling members may further be provided with breakout valves such that when the male and female coupling members are uncoupled the ends are sealed off preventing fluid from escaping.
However, releasable coupling arrangements comprising a socket and probe can create large separation forces, which act to expel the male and female coupling members apart. The separation force is generated due to the pressure of the fluid within the coupling assembly exerting a pressure upon the end portion of the male coupling member and is therefore a product of the pressure and cross-sectional area of the probe at the point where it exits the socket. Consequently, the separation force quickly becomes large with high-pressure fluids and large diameters. When the separation force becomes greater than the frictional force retaining the coupling parts together, it is necessary to incorporate an additional form of mechanical retention in order to prevent the coupling assembly from uncoupling.
However, such mechanical retention devices may be required to break under a predetermined force applied to the coupling assembly. For instance, for air-to-air refueling operations a tanker aircraft trails a fuel pipeline. At the end of the pipeline remote from the tanker aircraft is a drogue, which comprises the female coupling member. The aircraft to be refueled is fitted with a forward extending probe, the end of which forms the male coupling member. In order to prevent the coupling assembly from pulling apart during turbulence and with small changes in relative position of the aircraft, the coupling assembly must incorporate some form of retention means. However, in an emergency situation it is essential that the coupling releases under a predetermined force. This force is known as the breakout strength.
This desired breakout strength may be relatively low compared with the strength of the mechanical retention device used to overcome the separation force exerted upon the male member by the fluid within the coupling assembly. Consequently, this can lead to the retention device being constrained to only break or release under a higher applied force than would ideally be desirable, due to the design tolerances of the retention device.
The mechanism used to counteract the separation force of a coupling assembly can be separate from the mechanism used to provide the breakout strength, in order that the breakout strength may be set independently.
It is known to reduce the separation forces within coupling assemblies by arranging the assembly such that in addition to, and counteracting, the separation force created by the fluid, a force acting to resist separation is created by the fluid. The coupling assembly is arranged such that it comprises an internal surface upon which fluid exerts pressure of equal area to the cross sectional area of the male coupling member where it exits the female coupling member. Consequently the coupling is said to be “pressure balanced”, effectively resulting in a zero net separation force due to the internal fluid pressures.
Currently however, releasable coupling arrangements include protrusions and fluid paths that create turbulence when transmitting fluid between one conduit or pipe to another. It is also not possible to ‘pig’ such couplings, which is a requirement in the oil transmission industry and consists of moving a device through the inside of a pipe line for the purpose of cleaning, dimensioning, or inspecting.